1. Field of the Invention
The present invention relates generally to detecting events in a wireless sensor network, and more particularly, to detecting a complex event from various types of data obtained by multiple sensors.
2. Description of the Related Art
In wireless sensor networks, event detection plays key roles in most surveillance and monitoring application fields such as national surveillance, environmental monitoring and medical diagnostics. The above-mentioned events relate to data detected in different sensors, and a complex event may be defined as means for detecting real situations that are more complex than actually occurring events by integrating many different events described above.
Complex event detection requires multiple message communications since many different events are linked to each other in a complex way. Therefore, complex event detection has been developing to minimize message communication between sensors constituting the wireless sensor network and to detect the higher-order abstract real world.
For example, in order to detect real complex situations such as an explosion which cannot be detected by a single hardware sensor, a complex event may require various events detected in respective sensors, such as sudden changes in temperature, sudden changes in light, and sudden changes in noise levels. If there is an available sensor capable of directly detecting the explosion, it is possible to detect the explosion event by means of direct sensing without detecting the explosion by composing a complex event. However, when it is not possible to detect a certain event like an explosion simply by means of direct sensing, complex event detection is indispensable. Also, if there is a single sensor for detecting explosions, it is possible to handle much higher complex real-world events using the sensor.
Time window-based complex event detection in sensor networks is disclosed in “Event Detection Services Using Data Service Middleware in Distributed Sensor Networks”, by S. Li, S. Son, and J. Stankovic, Telecommunication Systems, Special Issue on Information Processing in Sensor Networks, vol. 26, no. 2-4, pp 351-368, August 2004.
The group-based real-time event detection services in sensor networks, proposed by S. Li et al., are applied to detect whether events constituting a complex event based on a reliability function have occurred within a time window set by developers. Complex events requiring an order of event occurrences have not been considered. The complex event of an explosion given as an example may be defined as a combination of individual events such as temperature, light and sound, and a reliability function has been defined that can give weights to the respective events and calculate their sum. When the sum exceeds a specific value determined by the application developers, a complex event such as an explosion is detected and reported.
Event detection is performed within a time window set by a given system or a client. With respect to the time window, if there is a difference in time for which different events last, event detection within the time window may fail to detect the required events or to rely on the reliability function set by the application developers.